Project Description: Mobility impairments in older adults decrease quality of life and are associated with high societal and economic burden. NIH RFA-AG-18-019 solicits applications ??to investigate the central neural control of mobility in older adults?using innovative and cutting-edge methods.? Current approaches to study the neural control of walking are limited by either the inability to measure people during walking (functional magnetic resonance imaging, fMRI) or the inability to measure activity below the cortex (functional near- infrared spectroscopy, fNIRS). We assert that a full and accurate understanding of the neural control of walking in older adults requires real time measurement of active regions throughout the brain during actual walking. We will achieve this by using innovative mobile brain imaging with high-density electroencephalography (EEG). This approach relies upon innovative hardware and software to deliver three-dimensional localization of active cortical and subcortical brain regions with high spatial and temporal resolution during walking. The result is unprecedented insight into the neural control of walking. Here, our overarching objective is to determine the central neural control of mobility in older adults by collecting EEG during walking and correlating these findings with a comprehensive set of diverse mobility outcomes (clinic-based walking, complex walking and community mobility measures). Our first aim is to evaluate the extent to which brain activity during actual walking explains mobility decline. In both cross sectional and longitudinal designs, we will determine whether poorer walking performance and steeper trajectories of decline are associated with the Compensation Related Utilization of Neural Circuits Hypothesis (CRUNCH). CRUNCH is a well-supported model of brain activity patterns that are seen when older individuals perform tasks of increasing complexity. CRUNCH describes the over-recruitment of frontoparietal brain networks that older adults exhibit in comparison to young adults, even at low levels of task complexity. CRUNCH also describes the limited reserve resources available in the older brain. These factors cause older adults to quickly reach a ceiling in brain resources when performing tasks of increasing complexity. When the ceiling is reached, performance suffers. The RFA also calls for proposals to ?Operationalize and harmonize imaging protocols and techniques for quantifying dynamic gait and motor functions?. In accordance with this call, our second aim is to characterize and harmonize high-density EEG during walking with fNIRS (during actual and imaged walking) and fMRI (during imagined walking). This will allow us to identify the most robust CRUNCH-related hallmarks of brain activity across neuroimaging modalities, which will strengthen our conclusions and allow for widespread application of our findings. Our third aim is to study the mechanisms related to CRUNCH during walking. Thus, our project will address a majority of the objectives in NIH RFA-AG-18-019 and will identify the neural correlates of walking in older adults, leading to unprecedented insight into mobility declines and dysfunction.